Spatially resolved spectroscopy across HD 189733A using exoplanet transits

Sammanfattning: For testing three-dimensional (3D) hydrodynamic models of stellar atmospheres, spectroscopy across spatially resolved stellar surfaces with high spectral resolution is desired. 3D models predict center-to-limb changes in asymmetries, shapes, strengths and wavelength positions of spectral line profiles, reflecting the hydrodynamics of the stellar atmosphere. However, except for a few supergiants and the Sun, current telescopes are not yet capable of resolving in detail any stellar surface. It is instead possible to exploit transiting exoplanets as probes of the atmospheres of their host stars. The spectra of the stellar surface portions covered by the planet during transit can be obtained by subtracting in-transit spectra from the stellar spectrum outside transit. Transiting planets typically cover only a small percentage of the stellar surface, requiring exceedingly high signal-to-noise ratios of the original spectra, obtainable by averaging numerous similar photospheric spectral lines. We apply this method to the 7.7 mag, K1 V star HD 189733A (‘Alopex’); its transiting planet covers ~ 2.5 % of the surface of its host star and is the deepest known transit among the brighter systems. Archival data from the ESO HARPS spectrometer are used to reconstruct spatially resolved representative profiles of photospheric Fe I lines, with the aim of comparing these to analogous synthetic line profiles computed from a 3D hydrodynamic model.